A radio communication system includes, as a minimum, a transmitter and a receiver. The transmitter and the receiver (which are often each part of combined transceiver unit) are interconnected by a radio-frequency (RF) wireless channel, which provides transmission of a communication signal between them. A receiver generally includes an amplifier, which is coupled to a receiving element (an antenna). The amplifier has a gain, which can be adjusted in a predetermined range, using a control signal. Many receivers also include a device which automatically adjusts the gain of the amplifier according to the level of the received signal. The process of adjusting the gain, according to which a received signal should be amplified, is called Automatic Gain Control (AGC). AGC circuits which are required to operate rapidly when they detect a signal are known in the art as fast attack AGC circuits.
In Time Division Multiple Access (TDMA) communication systems, an RF channel is shared among users attempting to access the radio system in certain of the time-division-multiplexed time slots. This enables transmission of more than one signal at the same frequency, allowing the sequential time-sharing of each channel by two or more users. The time slots are arranged in periodically repeating frames. Each of the frames includes a certain number of time slots and each of the slots provides a signal for a specified user. Nowadays, the signal is in a digital form.
TETRA (Trans-European Trunked Radio (also known as Terrestrial Trunked Radio)) is a system specified by the European Telecommunications Standards Institute (ETSI) in which a set of standards are laid down by which digital communications especially in a TDMA form are to take place in modern communications. In particular, TETRA Direct Mode Operation (DMO) (defined in European standard ETS 300-396-2), for example, for direct communication between users operates using 1:4 TDMA format. Each frame is divided into four time slots. Each receiver operating in this system receives a signal in only one of the four time slots per frame. Such systems require either receivers that have a dynamic range large enough to account for all signal levels and/or a receivers with a very fast AGC, which can adapt very rapidly to changing levels of received signals. The received signal has a preamble length of about 0.2 ms and the AGC response should be established during this period.
In particular, where DMO communications between two transceivers or mobile stations is carried out according to TETRA standard procedures, a receiver should be able to receive a DMO signal within a sensitivity level range of from −112 dBm to −20 dBm, i.e. 92 dB of dynamic range. In practice, signals can be in the dynamic range of from −112 dBm to 0 dBm. In addition, a DMO transmitter is permitted to have 6 dB overshoot at the beginning of the signal slot and the DMO receiver is required to be able to cope with this overshoot. This overshoot is additional to an overshoot that is usually caused by the circuit response of an AGC circuit to a step function at the beginning of a DMO signal slot. In view of these requirements an AGC circuit is required which gives improved fast attack performance compared with such circuits known in the prior art, a typical example of which is described in the following reference.
U.S. Pat. No. 5,742,899 to Blackburn et al., entitled “Fast Attack Automatic Gain Control (AGC) Loop for Narrow Band Receiver” is directed to a fast attack AGC loop having a first feedback loop with selectable response shapes and a second feedback loop with selectable response shapes. Response shape selection is based upon fast pull-down operation mode, overshoot recovery operation mode and steady state operation mode. The system described in the this reference is dedicated for operating in TDMA, and its response time is 1.5 ms for 25 kHz intermediate frequency baseband. The system has been optimized for the case when there is continuous transmission of RF power, thus allowing AGC settling to occur at the end of a time slot.
However, the prior art loop described in the said reference is not suitable for use in narrow band RF receivers or transceivers, e.g. for use in TDMA, in which the RF power is received in discontinuous bursts, such as in the TETRA Direct Mode Operation (DMO) because the response time of the loop is not sufficiently fast.
The Applicant's Copending EP Application No. 01116531.3 filed 9th Jul. 2001 (corresponding to U.S. Ser. No. 09/614668 filed 12th Jul. 2000) describes an AGC circuit which provides an improvement over the prior art. The purpose of the present invention is to provide a further improved AGC circuit for use in a radio communications receiver (transceiver).